In a typical downdraft gasifier, a vertically oriented gasification chamber receives feed stock (a fuel) which is combusted by pyrolysis in the gasification chamber. U.S. Pat. No. 6,647,903, incorporated herein by this reference, discloses a hollow cylinder with a lower cone-shaped section terminating in a restricted opening in order to retain the fuel in the gasifier and yet also allow spent fuel (char and ash) to exit the gasifier. In this way, fuel continually enters the gasifier and spent fuel continually exits the gasifier.
Steady fuel flow through a gasifier results in improved gasification, better gas quality, lower tar content, and higher yield. If the fuel flows too quickly through the gasifier, pyrolysis of the fuel is diminished and/or tar production increases. If the fuel flows too slowly through the gasifier, the fuel and/or char can build up in the gasifier and on the walls of the gasifier reducing gas quality. Gasifiers frequently suffer from bridging or channeling of the fuel. See Handbook of Biomass Downdraft Gasifier Engine Systems, Reed and Das, 1998 (Biomass Energy Foundation Press).
A gasifier with fairly steep long sloping walls defines a mass flow of fuel through the gasifier which provides an adequate flow of fuel through the gasifier but results in a gasifier with a restricted volume. The conical shaped gasifier thus must be rather tall and is not well adapted for use in portable, transportable, or on-site applications such as disclosed in co-pending U.S. patent application Ser. No. 12/070,032 incorporated herein by this reference.
A gasifier with short, less steep sloping walls at the outlet of the gasifier results in a funnel flow of the fuel. Fuel proximate the walls of the gasifier moves downward too slowly and interior fuel moves downward too quickly. Gasifier designs exhibiting a funnel-flow pattern results in material flowing preferentially through a funnel-shape channel located directly above the gasifier outlet while material outside this flow channel is stagnant. In some prior art designs, vibrators are provided for agitating the contents of the gasification chamber. See U.S. Pat. No. 7,736,402 incorporated herein by this reference.
There is a trade off, then, between the size of the gasifier and gasification parameters such as gas quality. For hoppers, mass flow and funnel flow are discussed in the paper “Solve Solids Flow Problems in Bins, Hoppers, and Feeders” by J. Marinelli and Dr. John W. Carson (June 2001), incorporated herein by this reference.